The present invention relates to a master cylinder used in a vehicle hydraulic brake system.
Many of today's motor vehicles are equipped with various automatic braking functions including traction control (TRC) and electronic stability control (ESC).
Some of vehicle hydraulic brake systems having one or more of such automatic brake functions are designed to draw hydraulic fluid (brake fluid) from the reservoir through a pressure chamber defined in the master cylinder for automatic braking.
It is desired that this type of hydraulic brake system have a master cylinder structured such that hydraulic fluid can flow from the reservoir into the pressure chamber without encountering any substantial resistance while the master cylinder is inoperative, and any fluid flow from the pressure chamber toward the reservoir during an initial stage of braking is restricted. A master cylinder that satisfies this requirement is disclosed in JP patent publication 2000-142365.
As shown in FIG. 6, this master cylinder includes throttle valve mechanisms 35 each including a floating valve body 36 and received in one of fluid passages 33 and 34 through which pressure chambers defined in a cylinder body communicate with the interior of the reservoir. When the pressure in the pressure chambers falls below the pressure in the reservoir, the valve mechanisms 35 are adapted to open, allowing hydraulic fluid to flow from the reservoir into the pressure chambers as indicated by chain line in FIG. 7.
When hydraulic fluid begins to flow from the pressure chambers toward the reservoir, the floating valve body of each valve mechanism will float in hydraulic fluid due to a pressure difference between the pressure chambers and the interior of the reservoir until pressed against a valve seat 38. After the valve mechanisms 38 have thus closed, hydraulic fluid flows only through a restricted passage 37 formed in the floating valve body 36 in a restricted amount as shown in FIG. 6.
Thus, in spite of the fact that hydraulic fluid can be smoothly supplied from the reservoir into the pressure chamber while the master cylinder is inoperative, as soon as the master cylinder is actuated, brake pressure is instantly generated in the pressure chamber, thereby minimizing the idle stroke of the piston at the initial stage of braking. Suitable reaction force is applied to the input members of the brake system due to the fact that hydraulic fluid flows through the restricted passage at a restricted flow rate. This prevents self-induced vibrations and noise produced at the initial stage of actuation of a negative-pressure booster (if the brake system has such a booster) due to insufficient reaction force. This in turn ensures smooth brake pedal feel.
As shown in FIGS. 6 and 7, the floating valve body 36, of which the initial position is determined by the cylinder body 31, is adapted to be seated on the valve seat 38, which is formed on the reservoir 32 at its portion 39 connected to the cylinder body 31. Thus, the master cylinder disclosed in JP patent publication 2000-142365 has the following problems.    (1) The stroke L of the floating valve body 36 tends to vary according to the dimension of a seal member disposed between cylinder body 31 and the connecting portion 39 or the degree of compression of the seal member, thereby destabilizing the operation of the throttle valve. This leads to variation in the performance of the master cylinder itself, thereby lowering its reliability.    (2) Since the valve mechanisms 35 of this patent publication are basically of the normally open type, small foreign objects that have infiltrated through the filter into the reservoir may deposit on the floating valve body 36. If such foreign objects get stuck between the valve body 36 and the valve seat 38, a gap may develop between the valve body 36 and the valve seat 38 that is even greater in sectional area than the restricted passage 37. Thus, when the brake pedal is depressed, hydraulic fluid can flow from the pressure chamber into the reservoir not only through the restricted passage 37 but through the gap present between the valve body 36 and the valve seat 38. This makes it difficult to sufficiently restrict the fluid flow from the pressure chamber into the reservoir.    (3) Fluid pressure applied to the restricted passage in the initial stage of braking (hereinafter referred to as “backflow pressure”) acts as a force to push up the reservoir, thereby detrimentally affecting the seal at the connecting portion 39, which in turn increases the possibility of leak of fluid through the seal.
An object of the present invention is to improve the reliability of a master cylinder including a throttle vale provided in a fluid passage between the reservoir and a pressure chamber to suppress backflow of hydraulic fluid into the reservoir at a low pressure, thereby improving pedal feel, by stabilizing the function of the throttle valve. Another object of the invention is to provide a master cylinder which is small in size of the connecting portion between the cylinder body and the reservoir in spite of the fact the master cylinder includes a throttle valve, and which is simple in structure and high in productivity.